Products constructed from carbon and/or graphite composites possess high thermal conductivity, significant heat capacity and excellent friction and wear properties. Because of these characteristics, such composites are often used in speciality applications ranging from the heat shields on the leading edges of the Shuttle Orbiter to exit cones for rocket engines. More commonly, however, these composites are used as frictional materials in the braking systems for military and large commercial aircraft where their unique characteristics provide significantly improved braking performance.
The manufacture of carbon and graphite composites is a lengthy process, and generally involves several cycles of densification and carbonization under substantially high pressure levels. The typical process begins by preparing a carbon preform using a hand lay-up of woven carbon fiber fabric or by hot pressing a mixture of chopped carbon fibers and resin. The preform is then densified by repeated liquid impregnation with pitch or resin, or by carbon vapor infiltration. Following densification, the preform is carbonized or graphitized by heating the preform to temperatures in excess of 600° C. and as high as 3000° C., as described by K. J. Huttinger, “Theoretical and Practical Aspects of Liquid-Phase Pyrolysis as a Basis of the Carbon Matrix of CFRC,” in Carbon Fibers, Filaments and Composites, 301, 301-326 (Figueiredo ed., Kluwer Academic Publishers, Boston 1990), and Brian Rand, “Matrix Precursors for Carbon-Carbon Composites,” in Essentials of Carbon-Carbon Composites, supra at 67-102. This densification/carbonization process is repeated until the desired density is achieved (normally 1.8 g/cc). Typically, the complete process requires up to five densification/carbonization cycles over a 6 to 9 month period. As such, products containing carbon or graphite composites tend to carry an extremely high cost.
Other methods of producing carbon or graphite composites have recently been developed so as to avoid the high costs and time expense associated with their manufacture. For example, U.S. Pat. No. 5,556,704, Prevorsek, et al., discloses a method for manufacturing carbon-carbon composites by hot pressing a mixture of carbon fiber and carbon precursor material. Essential to this process is the application of a uniaxial compressive force and a lateral restraining force to the mixture during the heating process. The compressive force is generated by a conventional press apparatus, while the lateral restraining force is generated by a hydraulic piston arrangement. Ultimately, the application of these pressure forces, along with other densification/carbonization processes, results in a high density-plate or rod structure.
The recently developed methods, however, are limited in that they require the use of a hot press and thus only allow the production of objects maintaining certain shapes suitable for extrusion or hot pressing, i.e., rods or plates. The manufacturer is therefore required to machine the resulting billet to achieve any product having a complex shape or structural feature, such as a threaded part or a turbine rotor. The additional machining, in turn, consumes more time and increases the final cost of the product.
The high cost of carbon and graphite composites has so for restricted their use to aircraft brakes and other relatively cost insensitive and/or performance driven applications. The benefits of these composites, however, may be readily transferred to the commercial sector if the cost of their manufacture was substantially reduced. For example, commercial applications may include clutch and braking systems for heavy trucks, or railroad locomotives and rail cars. The military may also find numerous applications in brakes and clutches on its fighting vehicles, such as tanks, armored cars, and self propelled artillery.
Affordable graphite or carbon objects having highly complex shapes are also desirable. These shapes may include the intricate designs of a turbine blade or a product having a threaded part. Current methods of manufacture, however, are unable to readily produce such objects. An ideal solution to this problem would include a new method which allows the casting of carbon and graphite composites in a mold such as that used in gelcasting technology.
Gelcasting is a traditional process for producing ceramic components having complex or intricate designs. Specifically, gelcasting is a method of molding ceramic powders into wet “green” products wherein a monomer solution serves as a binder vehicle, and its controlled polymerization in solution is used as a setting mechanism. The resulting green product is of exceptionally high strength and is typically dried to remove water. After drying, the product is normally heated further to burn off the polymer and is sometimes subsequently fired to sinter the product to a higher density.
Gelcasting methods are well known in the art and are described in Janney, U.S. Pat. No. 4,894,194, Janney et al, U.S. Pat. No. 5,028,362, and Janney et al., U.S. Pat. No. 5,145,908; A. C. Young et al. “Gelcasting of Alumina,” J. Am. Ceram. Soc., 74 [3] 612-18 (1991); (describing the gelcasting of ceramics such as alumina) Mark A. Janney et al. “Gelcast Tooling: Net Shape Casting and Green Machining,” in Materials and Manufacturing Processes, (1997) (describing the use of a water-based gelcasting system to form parts using H13 tool steel powder); S. D. Nunn et al., “Gelcasting of Silicon Compositions for SRBSN,” Ceram. trans., 62, 255-62 (1996) (describing the use of an alcohol-based gelcasting system and a water-based gelcasting system to form parts using silicon powder); M. A. Janney, “Gelcasting Superalloy Powders,” in P/M in Aerospace, Defense and Demanding Applications, 1995 (Metals Powder Industries Federation, Princeton, N.J., 1995) (describing the use of a water-based gelcasting system to form parts), which are all incorporated herein by reference.
The typical gelcasting process involves the formation of a slurry mixture including ceramic powder, a dispersant for the ceramic powder, and a monomer solution. The monomer solution includes one or more monomers, a free-radical initiator, and an aqueous solvent. Upon its combination, the slurry mixture is transferred to a mold where it is heat-treated at a temperature and for period of time sufficient to polymerize the monomer(s) and form a firm polymer-water gel matrix. The resulting green product is then heat-treated further to achieve a final ceramic product.
With a modification and refinement of the gelcasting process it is possible to extend gelcasting technology to the manufacture of carbon and graphite products. Accordingly, the limitations related to current methods for fabricating graphite and carbon composites can be virtually eliminated.